Reactive Spray Deposition Technology falls into a subset of deposition processes known collectively as thermal spraying. Thermal spraying and plasma spraying are both common deposition techniques used in the production of materials with controlled microstructure. Plasma spraying traditionally involves passage of a solid powder through or into a DC or AC plasma, subsequent melting of the solid particles and splats of material deposited on the substrate. The length of time the material spends in the plasma depends on the type of torch, gas flows and plasma shaping devices (i.e. cooling shrouds). Microstructure and spray efficiency are partially determined by torch design. Plasma processing is considered a high-energy technique. Alternatively, lower energy technologies have been explored as possible alternate deposition techniques to plasma spraying.
Several similar techniques for open atmosphere lower energy flame depositions have been developed to date. Listed below are some developments in thermal spray technology related to fuel cells:                1) Flame assisted vapour deposition (FAVD), in London at the Imperial College of London (UK-1995),        2) Oxy-acetylene combustion assisted aerosol-chemical vapour deposition (OACAACD), in China at the University of Science and Technology of China (China-2004),        3) Combustion chemical vapour deposition (CCVD) at MicroCoating Technologies, Georgia Tech, and North Carolina State University, (USA-1993),        4) Flame spray Pyrolysis in Zurich at ETH-Particle Technology Laboratory, (Switzerland-1998), and        5) Liquid Feed Flame Spray Pyrolysis at University of Michigan (USA-2004)        
The techniques listed above all relate to a generalized process involving pumping a dissolved metal-organic or metal-inorganic precursor through an atomizing nozzle and combusting the atomized spray. The atomization of the liquid can be accomplished by ultrasonics, air shear, liquid pressure, dissolved gases, heat or a combination of energy inputs. Precursor solutions containing the metal reactants required in the deposited film are pumped under pressure to the nozzle by use of a syringe or HPLC pump. In addition, some techniques feed the precursors to the combustion nozzle as an aerosol and the combustion nozzle is not used in the atomization process.
In some of the techniques, a dissolved gas is added to the precursor solution to aid in atomization. The droplet size and distribution has an impact on the final coating and is therefore important in the design/arrangement of the technique or type of atomizer. Regardless of the nozzle type, the atomized spray is then combusted by an ignition source such as a single pilot flame from a point source or a ring of pilots surrounding the exit of the nozzle. An optimal ignition point must be chosen since igniting too close to the exit of the nozzle results in a fuel rich mixture that does not burn easily while igniting too far away results in an oxidant rich mixture. Pilot gases consist of methane and oxygen, hydrogen or an oxy-acetylene type gas. Pilot gases are supplied to the system by mass flow controllers or by passive rotameters.
Depositions onto substrates usually occur by positioning the flame in front of or near the desired substrate and allowing the reaction to occur long enough for the desired thickness of film. The distance from the flame tip to the substrate influences the coating morphology, efficiency, boundary layer and the substrate temperature. If a nano-structured or dense film is desired then the flame should penetrate the boundary layer of the substrate. Longer flames (i.e. distance from nozzle to substrate) and higher concentrations of precursor material favour nucleation of particles and agglomeration instead of growth from the vapour phase (of a film) directly on the substrate. In other words, the droplets vaporize leaving the precursor material as a small gas vapour that then nucleates into a solid and then the solids agglomerate into larger particles. This process occurs from spray to flame tip and beyond. A powdery agglomeration of particles with poor adhesion occurs if the gap between the nozzle and the substrate is too large.
Care must be taken to prevent thermal shock to certain substrates by controlling the heat up and cool down to deposition temperatures when the flame is brought very close to the substrate. This is generally done by heating the substrate from the back by resistive heaters or by another flame.
Additionally, the heat-up and cool-down must be performed without the reactive precursors present so that a constant deposition temperature is maintained during film growth.
The above-listed techniques differ in some respects such as the method of atomization, type of atomizer, solution injection geometry and the fuel used in the flame. Summaries of the techniques are listed below.
Xu and colleagues (3) at NC State used a TQ-20-A2 Meinhard nebulizer for atomizing and a single point pilot flame for ignition of the atomized spray. In addition, a heating torch was applied to the back of the substrate holder to minimize the thermal gradient between the front and back of the substrate.
Meng et al (2) at the University of Science and Technology in China used a modified oxy-acetylene torch with a 2 mm diameter and fitted at an angle of 45° angle to the substrate. Precursors were supplied to the torch by means of an ultrasonic nebulizer injected directly into the torch. The oxy-acetylene flame core reaches temperatures as high as 3000 C. Unlike other versions of this technology, the flame is not produced by the precursor solvent but by an oxy-acetylene gas mixture. This process has been named oxy-acetylene combustion assisted aerosol-chemical vapor deposition (OACAACVD).
The system at nGimat (formerly MicroCoating Technologies) consists of a proprietary spray/combustion nozzle, the Nanomiser®, that functions on pressure and heat input for formation of very small droplets that are then combusted by a ring of methane/oxygen pilot lights. It is claimed that the specific geometry of the Nanomiser® allows for the formation of these small droplets which has not been attainable by other technologies. A precursor solution is delivered under pressure to the nozzle and heated prior to exit where a shear force is created by an unheated collimating gas.
Dr. Xu at NC State uses a system similar to nGimat, however the Nanomiser® nozzle has been replaced by a different off-the-shelf nebulizer.
Steele and Choy (1) at the Imperial College of London have been using a system of deposition named flame assisted vapor deposition (FAVD). The system was first reported in 1995 and work on SOFC cathode materials was published in 1997. The process consists of an air atomizing nozzle and a separate flame. The air atomizer is directed at a substrate on a hotplate and a separate flame is arranged perpendicular between the substrate and atomizer. The atomized spray passes through the flame and onto the substrate.
Flame Spray Pyrolysis (FSP) was developed at ETH in Switzerland by Dr. Pratsinis. A variety of products have been synthesized by FSP as for example silica, bismuth oxide, ceria, zinc oxide, zinc oxide/silica composites, platinum/alumina. Using this technique, a 35 cm spray flame produces 300 g/h of fumed silica using oxygen as dispersion gas. The particles are colleted in a baghouse filter unit.
SOFC/PEM (solid oxide fuel cell/proton exchange membrane) components can be fabricated via routes such as electrochemical vapour deposition (EVD), chemical vapour deposition (CVD), physical vapour deposition (PVD), sol-gel, RF-sputtering, spin coating, slurry spraying, plasma spray and screen-printing.
Various developments in the field of thermal spraying have also been presented in patent literature, e.g. U.S. Pat. No. 6,601,776 to Oljaca et al, U.S. Pat. No. 6,808,755 to Miyamoto et al., and US Patent Application 2005/0019551 to Hunt et al.
While all the above developments have some advantages, there is still a need for a low cost, rapid processing method that can be performed continuously, preferably without the need for long sintering times at elevated temperatures.